As the wireless communications industry blooms, the fast development of wireless transmissions brings in various products and technologies that are used in multiple-frequency transmissions. Thus, many products are equipped with the wireless transmission capability to meet consumer requirements. In addition, it is very important for a wireless transmission product to have a good antenna.
In general, conventional antennas can receive or transmit a signal of a specific frequency band. When a wireless transmission product receives an external signal, theoretically the antenna only receives the signal of the specific frequency and will not receive signals of other frequency. However, conventional antennas will produce multi-frequency signals and other unnecessary signals while receiving signals, and thus will cause noises and interferences to the posterior circuits. A common method for filtering out those unnecessary multi-frequency signals is by adding a filter to the posterior circuit. Nevertheless, not only such method will increase the cost of the circuit, but also the additional filter will occupy some space that is a shortcoming for the trend of pursuing miniaturized wireless transmission products. Therefore, the present invention provides an antenna with a filter to overcome the foregoing shortcomings.